Magnetic resonance imaging (MRI) apparatuses are apparatuses for irradiating a radio frequency magnetic field of a specific frequency on an object of measurement placed in a static magnetic field to induce magnetic resonance phenomenon and thereby obtain physical and chemical information of the object of measurement. The magnetic resonance imaging (MRI) method currently widely spreading is a method of imaging hydrogen nucleus density, difference of relaxation time, or the like, which differs depending on type of body tissue, by mainly using magnetic resonance phenomenon of protons in water molecules. Difference of tissues can be thereby imaged, and thus it is highly effective for diagnosis of diseases.
On the other hand, magnetic resonance spectroscopy (MRS) and magnetic resonance spectroscopic imaging (MRSI) are methods of separating magnetic resonance signals for every molecule on the basis of difference in resonance frequency (chemical shift) caused by difference of chemical bonds in the molecules (metabolites), and measuring density, relaxation time, or the like for every molecular species. MRS is a method of observing molecular species in a certain selected special region, and MRSI is a method of imaging every molecular species. The atomic nuclei used as the object include those of 31H (proton), 31P, 13C, 17F, and so forth.
Major metabolites existing in human bodies and detectable by proton MRS or proton MRSI (henceforth referred to simply as MRS/MRSI) utilizing protons as the objective nucleus species include choline, creatine, N-acetylaspartic acid (NAA), lactic acid, and so forth. It is expected to perform non-invasive stage determination or early diagnosis, and diagnosis of malignancy of metabolic disorders such as cancers, on the basis of amounts of such metabolites.
Since such metabolites existing in human bodies show signal intensity corresponding to only about 1/1000 of that of water molecules, weak signals from the metabolites are buried in the foot of the gigantic peak signal generated by water, and detection of metabolite signals is difficult. Therefore, there are methods of suppressing water signals in order to measure signals from metabolites. For example, there is a method of preliminarily suppressing water signals by using a radio frequency (RF) pulse having a frequency band similar to the frequency band of water signals, and detecting marginal signals of metabolites (refer to, for example, Patent document 1). The method of suppressing signals by pseudo saturation around the resonance frequency band of unnecessary signals is called CHESS (CHEmical shift Selective) method.
As described above, in order to measure metabolites, it is necessary to suppress water signals. However, measuring not only metabolite signals, but also water signals provides the following advantages.
(1) Correction of eddy current-induced distortion using phase of water signals:
By eddy currents generated at the time of application of the gradient magnetic field, phases of metabolite signals are changed and metabolite peaks are distorted. In order to correct such peak distortion due to the phase change, the phases are corrected by using water signals showing signal intensity higher than that of metabolite signals (refer to, for example, Non-patent document 1). The phase distortion is corrected by this correction of eddy current-induced distortion, and thus favorable metabolite peaks can be obtained.
(2) In vivo temperature measurement using resonance frequency of water:
The resonance frequency of water shifts depending on temperature, and the shift amount is represented with a temperature coefficient of −0.01 ppm/° C. On the other hand, it is known that the resonance frequencies of metabolites such as NAA do not change in the in vivo temperature range (refer to, for example, Non-patent document 2). It has been reported that in vivo temperature measurement is possible from frequency difference between water and metabolite by using the aforementioned characteristics (refer to, for example, Non-patent document 3). It is expected that in vivo temperature measurement may provide novel indexes for identification of ischemic region in chronic stage cerebral infarction, distinction of ischemic center and circumference region in acute stage cerebral infarction, and differentiation of tumor cytoma.